[英]LINQ many-to-many relationship, how to write a correct WHERE clause?
I use many-to-many relationship for my tables. 我为表格使用多对多关系。
There is a query: 有一个查询:
var query = from post in context.Posts
from tag in post.Tags where tag.TagId == 10
select post;
Ok, it works fine. 好的,它工作正常。 I get posts having the tag specified by id.
我得到的帖子有id指定的标签。
I have a collection of tag ids. 我有一组标签ID。 And i want to get posts having every tag in my collection.
我希望收到包含我的收藏品中每个标签的帖子。
I try the following way: 我尝试以下方式:
var tagIds = new int[]{1, 3, 7, 23, 56};
var query = from post in context.Posts
from tag in post.Tags where tagIds.Contains( tag.TagId )
select post;
It doesn't work. 它不起作用。 The query returns all posts having ANY one of the specified tags.
该查询返回具有任何一个指定标记的所有帖子。
I want to get a clause like this but dynamicaly for any count of tags in the collection: 我希望得到一个这样的子句,但动态地对集合中的任何标签数量:
post.Tags.Whare(x => x.TagId = 1 && x.TagId = 3 && x.TagId = 7 && ... )
You shouldn't project each post's tags in the outer query; 你不应该在外部查询中投射每个帖子的标签; rather, you need to use an inner query which performs the check for the outer filter.
相反,您需要使用内部查询来执行外部过滤器的检查。 (In SQL, we used to call it a correlated subquery .)
(在SQL中,我们曾经称它为相关子查询 。)
var query =
from post in context.Posts
where post.Tags.All(tag => tagIds.Contains(tag.TagId))
select post;
Alternate syntax: 替代语法:
var query =
context.Posts.Where(post =>
post.Tags.All(tag =>
tagIds.Contains(tag.TagId)));
Edit : Correcting per Slauma's clarification . 编辑 :纠正每个Slauma的澄清 。 The version below returns posts which contain, at least, all the tags in the
tagIds
collection. 下面的版本返回的帖子至少包含
tagIds
集合中的所有标签。
var query =
from post in context.Posts
where tagIds.All(requiredId => post.Tags.Any(tag => tag.TagId == requiredId))
select post;
Alternate syntax: 替代语法:
var query =
context.Posts.Where(post =>
tagIds.All(requiredId =>
post.Tags.Any(tag =>
tag.TagId == requiredId)));
Edit 2 : Corrected above per Slauma. 编辑2 :每个Slauma修正如上。 Also including another alternative making full use of query syntax below:
还包括下面充分利用查询语法的另一种选择:
// Project posts from context for which
// no Ids from tagIds are not matched
// by any tags from post
var query =
from post in context.Posts
where
(
// Project Ids from tagIds that are
// not matched by any tags from post
from requiredId in tagIds
where
(
// Project tags from post that match requiredId
from tag in post.Tags
where tag.TagId == requiredId
select tag
).Any() == false
select requiredId
).Any() == false
select post;
I've used .Any() == false
to simulate the NOT EXISTS
operator in Transact-SQL. 我使用
.Any() == false
来模拟Transact-SQL中的NOT EXISTS
运算符。
这实际上很容易做到:
var tags = context.Posts.Where(post => post.Tags.All(tag => tagIds.Contains(tag)));
Another option is to intersect the two lists if you want the collection of tags to ONLY contain the set you specify and no others: 如果您希望标记集合仅包含您指定的集合而不包含其他集合,则另一个选项是交叉两个列表:
var query = from post in context.Posts
let tags = post.Tags.Select(x => x.Id).ToList()
where tags.Intersect(tagIds).Count() == tags.Length
select post;
Try it with Any
. 尝试使用
Any
。
var query = from post in context.Posts
from tag in post.Tags where tagIds.Any(t => t == tag.TagId )
select post;
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